The present invention relates to a controller of a refrigerating plant, and more particularly to a controller adapted for application to a refrigerated transporting unit.
A system diagram for such a refrigerating plant is shown in FIG. 3.
A refrigerant gas at high temperature and high pressure that is discharged from a compressor 1 enters a condenser 2 where the refrigerant gas is condensed to be liquified by releasing its heat into outside air blown by a condenser fan 4. The liquefied refrigerant enters an electronic expansion valve (referred to as EEV hereinafter) 6 where it becomes a mixture of vapor and liquid phases as a result of an adiabatic expansion caused by a reduction of area. Then, the refrigerant in the vapor-liquid phases enters an evaporator 3 where the refrigerant is evaporated to be gasified by room air to be cooled and blown by an evaporator fan 5, and returns to the compressor 1.
The outputs of an intake refrigerant pressure sensor 9 for detecting the pressure of the refrigerant sucked into the compressor 1, an intake refrigerant temperature sensor 10 for detecting the temperature of the refrigerant and a blown air temperature sensor 8 for detecting the temperature of the air blown down from the evaporator 3, are input to a controller 11. By commands from the controller 11, the openings of the EEV 6 and a hot gas control valve (referred to as MV) 13 that is inserted in a hot gas bypass circuit 12 are regulated, and an electric heater 7 for heating air to be blown down from the evaporator 3 is turned on and off.
When the blown air temperature detected by the blown air temperature sensor 8 is lower than a set temperature that is preset in the controller 11, the refrigerating capacity is lowered by augmenting the quantity of the refrigerant gas that is bypassed through the hot gas bypass circuit 12 by increasing the opening of the MV 13 upon receipt of a command from the controller 11 in response to the deviation between the two temperatures.
When the opening of the MV 13 exceeds its set value for the upper limit (referred to as U-MV hereinafter), the control target value of the degree of superheat (value obtained by subtracting the saturation temperature corresponding to the pressure detected by the intake refrigerant pressure sensor 9 from the temperature detected by the intake refrigerant temperature sensor 10, referred to as SSH hereinafter) of the refrigerant gas at the outlet of the evaporator 3 is gradually raised to reduce the circulating quantity of the refrigerant and to further lower the refrigerating capacity by reducing the opening of the EEV 6.
In case the blown air temperature is still lower than its set temperature even after the adjustment of the openings of the MV 13 and the EEV 6, the operation is shifted to that of turning on the electric heater 7 (this operation is referred to as STEP 2 hereinafter). During the operation of STEP 2 and when the opening of the MV 13 is lower than the L-MV, in the event the blown air temperature does not go down even when the control target value of the SSH is set at the value for the highest capacity output, the operation is shifted to that having the electric heater 7 turned off (this operation is referred, to as STEP 1 hereinafter).
On the contrary, when the blown air temperature exceeds its set temperature, the refrigerating capacity is augmented by reducing the opening of the MV 13 in response to the deviation between the two temperatures. When the opening of the MV 13 falls below its lower limit set value (referred to as L-MV hereinafter), the refrigerating capacity is further augmented through an increase in the opening of the EEV 6 by gradually lowering the control target valve of the SSH.
However, in the conventional device, when the opening of the MV 13 exceeds the U-MV, the opening of the EEV 6 goes down so that the circulating quantity of the refrigerant is decreased. Then, the refrigerant is evaporated in the portion on the inlet side of the evaporator 3, with air that passes through the portion on the inlet side alone being cooled, whereas air that passes through other portions is not cooled. This gives rise to an inconvenience that the temperature distribution of the air that is blown down from the evaporator 3 becomes unsatisfactory, which results in an aggravation of the temperature distribution within the refrigerated chamber.
In order to cope with the situation, STEP 2 is executed by turning on the electric heater 7 despite the fact that of the conditions may allow the execution of STEP 1, and thus there is a disadvantage that the power is wastefully consumed.